Surge arrester

ABSTRACT

A surge arrester comprising a stack of a plurality of cylindrical varistor blocks (10), preferably made of metal oxide, which are arranged one after the other in the axial direction of the varistor blocks between an upper end electrode (11) and a lower end electrode (12) and surrounded by an elongated, electrically insulating external casing (19) of rubber or other polymeric material. The end electrodes (11, 12) are interconnected by means of at least three clamping members (15) of insulating material to achieve the required contact pressure between the different elements (10-14) in the stack. Between the lower end electrode (12) and the first block (10) in the stack of varistors, there is arranged a pivot means comprising a centrally placed pivot member (17), projecting from the lower end electrode (12), said pivot member (17) making contact with a pressure plate (14), resting against the lowermost block (10) in the stack of varistors, to achieve articulation in all transverse directions.

FIELD OF THE INVENTION

The present invention relates to a surge arrester comprising a stack ofa plurality of cylindrical varistor blocks which are preferably made ofmetal oxide and are arranged one after the other in the axial directionof the varistor blocks between two end electrodes and surrounded by anelongated, electrically insulating outer casing of rubber or otherpolymeric material. The required contact pressure between the variousinstruments in the stack is provided achieved one or more clampingmembers extending between the two end electrodes and secured thereto.

DESCRIPTION OF RELATED ART

Surge arresters of the above-mentioned kind are previously known fromthe patent documents U.S. Pat. No. 4,656,555, U.S. Pat. No. 5,291,366and EP-A-0 230 103. A drawback with these known designs is that theyexhibit little resistance to mechanical influence in a directiontransverse to the surge arrester.

To carry large currents, a sufficient contact pressure must be achievedbetween the blocks in the stack. In the known devices, this contactpressure is achieved by prestressing the varistor stack with an externalmechanical, electrically insulating joint. The varistor stack is verystiff in relation to the prestress elements, and a transverse externalload, applied to the upper end electrode, is absorbed as a bendingstress in the varistor stack. This bending stress entails a forcedistribution over the surface of the varistor blocks which provides acompressive stress, increasing towards the edge, in the direction ofdeflection and a corresponding pressure relief in the oppositedirection. Such pressure reliefs give rise to insufficient contactpressure and cannot, therefore, be accepted. A known solution to thisproblem is to increase the prestressing force such that sufficientcontact pressure is obtained over the whole surface of the varistorblocks. However, the varistor blocks are brittle, so they can easilycrack as a result of great compressive stresses at the edges. The knownsolutions therefore strike a balance between maintaining a sufficientcontact pressure and not exceeding the strength of the varistor blockswith respect to bearing pressure.

A surge arrester of the kind described above may alone constitute theactive part in a surge arrester for medium-voltage systems. A pluralityof surge arrester may also, like modules, be connected together into acomposite (series-connected) surge arrester intended for higher systemvoltages. When transverse load bears on such a surge arrester composedof several modules, a bending moment arises over the entire compositesurge arrester. The varistor blocks in the lowermost arrester module arethereby subjected to very great compressive stresses and tensilestresses, respectively. A further drawback with these known designs is,therefore, that the resistance to external transverse forces is greatlyreduced when joining surge arrester modules together.

SUMMARY OF THE INVENTION

The invention provides a surge arrester of the above-mentioned kindwhich has better resistance to external, transverse loads than prior artarresters. This is achieved according to the invention by strap-shapedprestress elements secured to the end electrodes and by a pivot meansbetween the varistor stack and the lower end electrode. The prestresselements are arranged such that the end electrodes are connected to eachother at at least three points, such that, in all directions, a bendingmoment caused by deflection is absorbed as tensile and compressiveforces, respectively, in the prestress elements. Also, an externalbending moment, attacking the upper end electrode, will be absorbed astensile and compressive forces in the prestress elements. A surgearrester composed of a plurality of modules is therefore capable ofwithstanding considerably greater external transverse forces than acorresponding surge arrester composed of the known design.

A surge arrester module with a pivot means which is loaded with anexternal transverse force gives rise to a bending moment in the upperend electrode. This leads to the creation of a mechanical stressdistribution over the surface of the uppermost varistor block, which isof the same magnitude as in the case without a pivot, but directed inthe opposite direction. In the lower end of the varistor stack, nomoment can be transmitted in case of an ideal pivot, and therefore nostress distribution arises over the lowermost varistor block. Byconstructing the pivot means such that it is partially capable oftransmitting a bending moment, an additional advantage is obtained. Inthis case, a bending moment can be transmitted from the lower endelectrode to the lower part of the varistor stack, which bending momentis directed in the opposite direction of the moment in the upper part ofthe varistor stack. The bending moments thus arising may be dimensionedto balance each other such that a considerably lower bearing pressureover the surface of the varistor blocks arises. Greater externaltransverse loads may thus be withstood.

The solution described above may be achieved with the pivot meansconsisting of an elastic plate. According to the invention, a pivotmakes contact with the pressure plate, the pivot being formed with aplane surface towards the pressure plate. This results in an additionaladvantage in that the pivot point, when the surge arrester is deflected,is displaced in the deflected direction, whereby the torque arms to theprestress elements are changed. The torque arm of to the prestresselement which is under tensile load becomes longer during thedeflection, which results in a lower tensile load in the prestresselement. In this way, the surge arrester is also given an initialstiffness, which means that a certain bending moment must be overcomebefore a greater deflection occurs.

The prestress elements may consist of straps, continuously wound ofglass-fibre strand and embedded into polymer. The straps are clampedonto shoulders projecting from the end electrodes, for example as shownin the German patent application P 43 06 691 7. Through the pivot means,the surge arrester will have a larger deflection amplitude at transverseforces than in prior art designs. This means that, upon deflection, thestraps resting against the shoulders projecting from the lower endelectrode are subjected to an unfavorable force distribution in thedirection of deflection. Upon such a deflection, the end electrodes arenot only displaced in parallel, but they are also positioned at an angleto each other. The displacement and the angular adjustment mean that across section of a strap in an axial plane parallel to the direction ofdeflection will become subjected to different forces at the inner andouter edges of the cross section. The edge load thus arising becomesdimensioning for the total load-absorbing ability of the strap. Aproblem then arises in that the load-absorbing ability of the strap isreduced if, at the same time, deflection is to be allowed.

According to the invention, the above-mentioned problems are solved bypivoting a lower load-absorbing part of the shoulder from an upper partof the shoulder, integrated with the end electrode, by means of a jointin the tangential direction. That part of the shoulder which makescontact with the strap then has a force transmission which is evenlydistributed in relation to the cross section of the strap. The force canthen be transmitted in a torque-free manner to the fixed part of theshoulder through the joint, which may consist of a rounding of the lowerpart of the shoulder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by description ofembodiments with reference to the accompanying drawings, wherein

FIG. 1 is a three-dimensional picture of a surge arrester according tothe invention with part of the casing of the arrester being cut away,

FIG. 2 shows such a surge arrester in an axial section, and

FIG. 3 shows an alternative embodiment of the lower part of the surgearrester.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The surge arrester modules shown in FIGS. 1 and 2 comprise a stack of anumber of varistor elements 10 in the form of cylindrical ZnO blocks.The varistor stack is clamped between an upper end electrode 11 and alower end electrode 12 with intermediate pressure plates 13, 14. The endelectrodes and the pressure plates may be made of a electricallyconducting material, for example aluminium. The axial compression of thevaristor stack is achieved with the aid of four electrically insulatingstraps 15, which are wound of continuous glass-fiber strand with aplurality of turns and embedded into thermosetting resin. The straps aresecured to the end electrodes, which are provided with four radiallyprojecting shoulders 16 with cylindrical contact surfaces. The strapsmay be prefabricated and then be clamped on the stack composed ofvaristor blocks, washers and electrodes by tightening a screw 17 whichis screwed into the lower end electrode and which at the same timefunctions as a joint screw or terminal.

The upper end electrode of the arrester module is provided with athreaded hole 18 for a screw to be screwed (series connection) to asimilar module or for external connection. Each end the surge arresteris provided with an end yoke 20 comprising four lugs 21 arranged on awasher, each lug overlapping a shoulder 16 and making contact with aprojecting support 22 at each shoulder. The lugs 21 reduce thedeflection of the surge arrester and counteract lateral contractionforces in the straps 15. The end yokes 20 also transmit a torque whenscrewing together the surge arrester modules or the end connection. Thesurge arrester module is provided with a casing 19 applied by casting,preferably an elastomer, for example silicone rubber or ethylenepropylene terpolymer (EPDM rubber).

FIG. 3 shows an alternative embodiment of the lower end electrode 12.The shoulders 16 projecting from the end electrode each comprise anupper fixed part 16a, integrated with the end electrode, and a lowerpivoted part 16b which comprises the semicircular contact surface facingthe strap 15. In a normal plane to the surge arrester, on a level withthe support 22, the fixed part 16a is formed with a plane contactsurface 30. The pivoted lower part 16b of the shoulder 16 is, in thesame plane, formed with a cylindrical contact surface 31 resting againstthe contact surface 30, which contact surface 31 has a directiontangential to an axial plane through the center of the shoulder. In thisway, the contact surfaces 30 and 31 form a joint through which forcesfrom the strap 15 may be transmitted in a torque-free manner to the endelectrode 12. For this reason, no uneven load of the cross section ofthe strap occurs when deflecting the surge arrester. The plate 14abutting the screw 17 differs from the preceding example in that itsedges are concave.

According to an advantageous development of the invention, the pivotmeans is made so stiff that it is able to partially transmit a bendingmoment. The bending moment arising at the lower end of the varistorstack can be dimensioned to partially counteract the bending moment atthe upper end of the varistor stack. Through this design, the surgearrester can take up considerably greater transverse forces than in theknown devices, and without exceeding the allowable bearing pressure inthe varistor blocks. This may be achieved by replacing the pivot meanswith an elastic plate, inserted between the pressure plate 14 and theend electrode 12, with a modulus of elasticity corresponding to a fewhundred MPa. When an insulated foot is desired, the elastic plate may bemade of an electrically insulating, elastic material. In thisembodiment, the electrical connection may be connected to the pressureplate 14.

The property of being able partially to transmit a bending moment mayalso be achieved by forming the screw 17, which is arranged through theend electrode 12, with a plane contact surface. The plane contactsurface of the screw must then be given a sufficient diameter, so that asmall torque arm is formed from the center to the edge of the screw, bywhich torque arm it is possible to transmit part of the external bendingmoment Lo the varistor stack. In this way, the pivot point is laterallyadjusted in the direction of the deflection, whereby the torque arms tothe straps 15 are favorably influenced such that smaller tensile forcesarise in the straps 15.

We claim:
 1. A surge arrester comprising:an upper electrode; a lowerelectrode; a plurality of cylindrical varistor blocks stacked in theaxial direction of said blocks between said upper and lower electrodes;at least three straps of continuous strand interconnecting said upperand lower electrodes to achieve a predetermined contact pressure in thestack; a pivot means arranged between a lowermost block in the stack andsaid lower electrode, said pivot means comprising a pivot membercentrally placed and projecting from said lower electrode and a pressureplate resting against said lowermost block, said pivot member contactingsaid pressure plate; and an electrically insulating casing surroundingsaid blocks.
 2. A surge arrester according to claim 1, wherein the pivotmember comprises a screw for prestressing the stack of varistors.
 3. Asurge arrester according to claim 1, wherein the pivot member comprisesat least one lining.
 4. A surge arrester according to claim 1, whereinthe pressure plate is formed with oblique edges.
 5. A surge arresteraccording to claim 4, wherein the pressure plate is formed with concaveedges.
 6. A surge arrester according to claim 1, wherein the pivotmember is formed with a plane surface making contact with the pressureplate.
 7. A surge arrester according to claim 1, wherein the pivotmember or the pressure plate is formed with a convex contact surface. 8.A surge arrester according to claim 1, wherein the pivot means furthercomprises an elastic plate arranged between the pressure plate and theend electrode.
 9. A surge arrester according to claim 8, wherein theelastic plate is electrically insulating and the pressure plate isprovided with an electric connection member.
 10. A surge arresteraccording to claim 1, wherein the end electrodes are provided withradially projecting shoulders, with which the clamping members makecontact, wherein each shoulder radially projecting from the lower endelectrode comprises a fixed part, integrated with the end electrode, anda pivoted part in relation to the fixed part, whereby the joints areable to transmit, in a torque-free manner, compressive stresses from theclamping members to the end electrode.
 11. A surge arrested according toclaim 1, wherein at each end electrode an end yoke is arrangedcomprising lugs, whereby the deflection of the surge arrester can bereduced, the lateral contraction effect of the clamping members can becounteracted and, when joining surge arrester modules, torque can betransmitted.